Conventionally, devices for unstacking flat objects essentially comprise a magazine for storing a stack of postage items and an unstacking head at the end of the magazine. The magazine includes drive means for presenting the first item in the stack in a well-defined position and standing on edge. The unstacking head may be constituted by a suction cup which performs a plunging movement to come into contact with the first item in the stack and to grasp it by suction, and which is moved in translation in order to take the item away from its stopped position to an exit position where it is taken up by the destination machine. During this time, the stack advances in the magazine and the next item takes the place of the preceding item.
British patent specification GB-A-2 125 367 illustrates this type of equipment. Although it includes a bellows, the suction cup is relatively rigid and only its lip adapts to the shape of the surface of the object to be grasped.
Suction cup systems are known in other fields where the problem of separating flat objects one-by-one does not occur in the same way (cf. the following patent documents: U.S. Pat. No. 3,724,687, DE-A-3 326 552, U.S. Pat. No. 3,193,281, EP-A-0 212 865, DE-C-1 112 538).
Postage items can be very varied in nature, and the surfaces they present to the unstacking head may be stiff or floppy, smooth or porous, and the item as a whole may be thin or thick, light or heavy. The unstacking head must be capable of grasping and moving thick and heavy items having stiff and smooth surfaces as well as open items; its suction force must therefore be sufficiently high. However, in a given batch of postage, there may also be light single sheets which are thin and porous and which have a natural tendency to adhere to one another so that the high suction force of the unstacking head tends, from time to time, to pick them up together and move them together to the exit position.
Unstacking, i.e. taking the items from the stack one-by-one thus remains a problem which is not fully solved.
The object of the present invention is thus to provide an improved unstacking head which is considerably better at separating thin objects, in particular, without degrading its performance when unstacking other objects.